In history class, the girl to boy ratio is $2$ to $5$. If there are $20$ boys, how many girls are there?
Answer: A ratio of $2$ girls to $5$ boys means that there is a group of $2$ girls for every group of $5$ boys. If there are $20$ boys, then there are $4$ groups of $5$ boys each. Because we know the ratio is $2:5$ , there must be $4$ groups of $2$ girls each. There is a total of $8$ girls in history class.